Abstract 918: Pre-B-cell Leukemia Homeobox Interacting Protein 1 is a Novel Regulator of Growth Signaling in the Heart

Abstract

The cellular mechanisms that allow for cardiomyocyte growth in different directions may yield important information about the development of hypertrophic and dilated cardiomyopathies, diseases which include the widening and lengthening growth of cardiomyocytes, respectively. We have previously found that genetic manipulations of the mitogen-activated protein kinase kinase 1 (MEK1) and extracellular signal-related kinase 1/2 (ERK1/2) signaling pathway cause cardiac hypertrophy and cardiomyocyte widening when the pathway is activated or dilated cardiomyopathy and cardiomyocyte lengthening when it is inhibited. The downstream effectors of the directional growth response controlled by these kinases have yet to be studied. Therefore, an unbiased phosphoproteomic screen was conducted on cardiomyocytes subjected to activation or inhibition of MEK1-ERK1/2 signaling. After validation of the screening hits with western and Phos-tag blots, Pre-B-cell leukemia homeobox interacting protein 1 (PBXIP1) emerged as an interesting target. Although it is known as a microtubule binding protein and regulator of proliferation in cancer cells, its function has never before been studied in the heart. Inhibition of MEK1-ERK1/2 increases phosphorylation of PBXIP1. However, activation of the pathway increases PBXIP1 protein expression, and as such we sought to determine the effects of PBXIP1 overexpression in the heart via adeno-associated virus 9 (AAV9) treatment. At four months of age, mice overexpressing PBXIP1 have significantly larger hearts than controls without a change in cardiac contractility. Intriguingly, histological analysis showed that cardiomyocytes highly infected with PBXIP1 AAV9 are significantly smaller in size compared to uninfected cells. Current studies are manipulating the PBXIP1 phosphorylation sites identified in the phosphoproteomic screen to determine the exact roles they may play in the cardiac growth response.